1. Field of the Invention
The present invention relates to a sheet carrying device for carrying sheet on a carrying belt in the state that the sheet is caused to adhere to the carrying belt by sucking power of a suction fan.
2. Description of the Related Art
In a stencil printing machine, which is an image forming machine, it is necessary to feed paper in a paper feed section to a printing section and discharge the paper on which images have been printed in the printing section to a paper discharge section. For such paper-discharge, for example, a paper carrying device in a suction manner is used.
FIG. 1 illustrates an outline of a paper carrying device of this type. As shown in FIG. 1, pulleys 1 and 2 are located between a paper carrying start position and a paper carrying end position. An endless carrying belt 3 is set up onto this pair of the pulleys 1 and 2 to stretch therebetween. One of the pulleys 1 and 2 is driven and rotated by a belt driving motor 4. By the rotating power of the belt driving motor 4, the carrying belt 3 is moved. A great number of non-illustrated air permeating holes are made in the carrying belt 3. A suction fan 5 is positioned below the carrying belt 3. This suction fan 5 is made so as to suck air above the fan 5 by driving power of a motor 5a for the fan.
In the above-mentioned structure, the carrying belt 3 is moved by driving power of the belt driving motor 4, and the suction fan 5 is rotated by driving power of the motor 5a for the fan 5. When the paper 6 on which images have been printed is loaded from the paper carrying start position onto the carrying belt 3, the paper 6 is caused to adhere to the carrying belt 3 by sucking power of the suction fan 5 and the paper 6 in this adhesive state is carried to the paper carrying end position.
In the above-mentioned conventional paper carrying device, however, the sucking power of the suction fan 5 is constantly made a fixed value regardless of the size of the paper 6, the basis weight thereof and the like. Therefore, in the case that the carried paper 6 is so firm and heavy as cardboard or is small and thin, more sucking power than is required is acted. As a result, there arise such problems that excessive electric power is consumed and excessive load is applied to the carrying belt 3 by excessive sucking power.